Double click on the gfx.font_rendering.directwrite.enabled entry so that it's set to true.

Double-click on the mozilla.widget.render-mode entry and set the value to 6. If you can't find this key, right-click anywhere in the window, hit to New > Integer, set it's name to mozilla.widget.render-mode, and value to 6.

Restart Firefox

Now, you should notice that your fonts look a little bit nicer, and certain pages (like Google Maps) move a little bit smoother. To revert back, just go back into about:config, disable gfx.font_rendering.directwrite.enabled, and set mozilla.widget.render-mode to -1. Hit the link below to read more.